Conventionally, there have been known an electronic component and a printed circuit board in which wiring patterns are formed on the upper and lower surfaces of a base material and in which these wiring patterns are connected through the base material.
And, to form such a structure, there have been proposed and disclosed various producing methods of forming a via hole in a base material, and then forming a conductor part in the via hole. FIGS. 6A, 6B, and 6C are explanatory sectional views illustrating a conventional method of forming a via hole and a conductor part.
As shown in FIG. 6A, conventionally, to form a via hole 2 in a base material 1, the base material 1 in which the via hole 2 is to be formed is set first on a stage (not shown). From above the base material 1 set on the stage, a drill 3 with an outer diameter corresponding to the hole diameter of the via hole 2 is lowered to perform drilling, thereby forming the via hole 2. When forming a plurality of via holes 2 in the base material 1, the stage or the drill 3 is translated along the surface of the base material, and drilling is repeated.
After the via hole 3 has been thus formed in the base material 1, a conductor part is formed in the via hole 3. FIG. 6B shows a first method of forming the conductor part; according to FIG. 6B, after the via hole 2 has been formed in the base material 1, the via hole 2 is filled with a conductive paste 5 containing a powdered low resistance metal, such as gold, silver, copper, or aluminum by using a squeegee 4 to thereby form a conductor part 6.
FIG. 6C shows a second method of forming a conductor part; according to FIG. 6C, after the via hole 2 has been formed in the base material 1, a metal layer 7 is formed on the inner wall surface of the via hole 2 by electroless plating, and this metal layer is used as the conductor part 6.
However, due to the reduction in via hole diameter as a result of a demand for a reduction in the size of electronic components, the method of forming a via hole and the method of forming a conductor part in a via hole, described above, involve the following problems.
The method of forming a via hole has a problem in that when the drill diameter is reduced as a result of a reduction in the via hole diameter, the machining with the drill becomes rather difficult, and the service life (work life) of the drill is shortened.
The method of forming a conductor part by filling a via hole with a conductive paste has a problem in that when the via hole is reduced in diameter, it becomes rather difficult to fill the via hole with the conductive paste by using a squeegee. Further, the conductive paste contains resin in addition to the metal powder, so that the resistance value of the conductor part increases, with the result that the heat dissipation property deteriorates, and a reduction in resistance cannot be achieved. The smaller the inner diameter of the via hole, the more serious these problem become.
Further, also in the method in which a conductor part is formed by electroless plating, the via hole is not filled with metal but involves generation of voids, so that, as in the method using a conductive paste, the resistance value of the conductor part increases, with the result that the heat dissipation property deteriorates, and a reduction in resistance cannot be achieved.
To solve the above-mentioned problems, a method is known in which a via hole is formed by laser irradiation and in which metal is deposited in the via hole by electroplating to form a conductor part (metal conductor) (see, for example, Patent Document 1).
<Patent Document 1>
Japanese Patent Application Laid-Open No. 2001-144444 A (Claim 3, FIG. 1)
However, in the method in which a via hole mentioned above is formed by laser irradiation, and a metal conductor is then formed, there occurs the following problem.
That is, in the method in which a conductor part is formed in a via hole by electroplating, the deposition rate is higher than that in the case of electroless plating, and while it is possible to form a conductor part in the via hole in a short time, the conductor part formed by deposition by the electroplating has a problem in that the bonding strength at the via hole interface, that is, between itself and the base material end surface. Thus, when an external force is applied to the base material, or when the base material repeatedly undergoes dimensional fluctuation due to changes in temperature and humidity, separation occurs at the interface due to the stress caused by the dimensional fluctuation of the base material, so that there is a fear of the conductor part being detached from the via hole.
In the case of the processing by laser irradiation, the hole has a trapezoidal sectional configuration, and a tapered portion is formed, so that not only a force in the shearing direction but also a force in the separating direction is applied to the interface, thereby making the separation more conspicuous.